The present invention relates to automatically controlled cutting machines for cutting limp sheet material such as fabrics, paper, plastics, leathers, rubber and the like. More particularly, the present invention is concerned with vacuum beds that are utilized to firmly hold the sheet material in position as cutting takes place.
In prior art machines for cutting limp sheet material, vacuum has been used to hold the material firmly in place during cutting. U.S. Pat. No. 3,495,492, having the same assignee as the present invention, discloses such a machine. The sheet material is covered with an air-impermeable overlay to reduce the amount of air drawn through the material into the vacuum bed and generate higher holddown forces which compact the material. The vacuum system, however, must have a capacity sufficient to exceed the leakage rate through the overlay and material when cut and, at the same time, maintain adequate holddown forces.
To minimize the capacity of vacuum systems used in cutting tables, the vacuum beds were sectionalized, and valves were employed to generate the vacuum only in those sections of the bed where the cutting blade was operating. Customarily, the vacuum bed in a cutting table is comprised by bristled mats or blocks such as shown in U.S. Pat. Nos. 3,765,285 and 4,205,835. Bristles exhibit limited resistance to air flow in directions perpendicular to the support surface and also parallel to the support surface. Thus, if vacuum is applied in one section of a bed, air flow from the support surface overlying the section and also from adjacent sections is drawn through the vacuum system.
To reduce the capacity of the vacuum system due to horizontal air flow through the bed from adjacent sections, expendable panels or separators were inserted in the bristles between each section as shown in U.S. Pat. No. 3,765,289. Such panels were made of paper, rubber, thin plastic and the like, and were cut each time the blade passed between the different sections in a cutting operation. With extended use, the panels were destroyed and lost their effectiveness in restricting horizontal flow and thus required periodic replacement.
It is accordingly a general object of the present invention to provide a vacuum bed for holding limp sheet material with different sections of the table separated by panels that are not destroyed during cutting. More particularly, it is an object of the invention to provide a vacuum bed having panels which restrict horizontal flow of air through the bed during most of the cutting operation and which are withdrawn from the bed as the blade moves from one section of the bed to another.